1. Field of the Invention
The present invention relates to a method of judging hydrogen embrittlement cracking of a material used in a high-temperature, high-pressure hydrogen environment and, more particularly, to facility management and prediction of life of a large-sized reactor used in petroleum refining equipment.
2. Related Art (Past Method)
Since heavy walled reactors used in a petroleum refining plant are operated in a high-temperature, high-pressure hydrogen environment, there is a danger that the susceptibility of catastrophic failure is increased by hydrogen. With respect to hydrogen embrittlement cracks and damages to pressure vessels, evaluation techniques have been explained to some extent in Japan and foreign countries standards. For example, as shown in FIG. 12, a pictorial diagram of the crack growth rate due to hydrogen embrittlement appears in Non-Patent Reference 1 that is an international recommended practice giving a guide in evaluating the soundness of a pressure vessel. This diagram is a qualitative expression of crack growth in a case where a crack has occurred in a pressure vessel that has been hydrogen embrittled. The horizontal axis indicates the stress intensity factor applied to the crack tip, while the vertical axis indicates the crack growth rate. The meanings of the symbols used in the diagram are as follows.
KIH: critical stress intensity factor for crack initiation (MPa√{square root over (m)})    KIC-H: fracture toughness of hydrogen-absorbed material indicated by fast fracture (MPa√{square root over (m)})    KIC: fracture toughness of hydrogen-free material (MPa√{square root over (m)})    da/dt: growth rate of hydrogen-assisted crack (mm/sec)
Besides, mathematical formulas for finding the stress intensity factor by simple calculations have been proposed. The mathematical formulas are shown below. The stress intensity factor is calculated by the following procedure and mathematical formulas.
1) Selection of Applied Stress (σ)
a) Applied stress=0.2% YS
b) Applied stress=circumferential (axial) stress+used stress+residual stress
circumferential stress=internal pressure×(inner diameter+wall thickness)/(2×wall thickness)
axial stress=internal pressure×(inner diameter+wall thickness)/(4×wall thickness)
2) Finding a flaw Shape Parameter (Q)Q=φ2−0.212×(σ/0.2% YS)2where φ is calculation of a shape parameter using elliptical integration by the Simpson's formula3) KI (Stress Intensity Factor)
embedded crack: applied stress (σ)×SQR (π×crack depth (a)/flaw shape parameter (Q))
surface crack: intrinsic crack KI×1.1
Reference: API Recommended Practice 579
The evaluation method by the above-described procedures, however, has the problems that it is difficult to make quantitative evaluations, and that the evaluations is too conservative. Therefore, there is a strong demand for establishment of a flaw evaluation method that has high reliability and can make quantitative evaluations.
However, where a quantitative evaluation is made, it is difficult to estimate residual stress existed in a pressure vessel. There is the problem that a huge amount of calculations using a computer is generally necessary for analysis of heat flow and residual stress analysis.
Furthermore, the aforementioned simple calculation method can calculate stress intensity factors without needing a large-capacity computer. However, to achieve a straightforward method, residual stress is contained in the stress in work. The yield stress of the material has been adopted as the stress in work. Using the value of the yield stress of the material means that the life of the pressure vessel is evaluated too conservatively. Therefore, there is the problem that this simple calculation method does not provide high reliability of evaluation.